The new data provide information on the levels of market concentration involved in the FTC’s investigation of 264 mergers, covering 1,372 markets over a 16-year period. The data tabulations use two statistics from the agency’s Horizontal Merger Guidelines to measure market concentration: 1) the post-merger Herfindahl-Hirschman Index (HHI), and 2) the change in the HHI, which reflects changes in market concentration before and after a particular merger. They also track the number of “significant competitors” in the markets in these cases. Data on HHIs are available for 1,359 markets and data on significant competitors are presented for 1,143 markets. The HHI reflects the overall composition of the market and the distribution of market shares of the competing firms.

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